Transparent zirconia sintered bodies have a high refractive index and a high permittivity and hence are expected to be utilized as optical members (patent document 1). In particular, transparent zirconia sintered bodies to which titania has been added have a high transmission and, hence, application thereof to optical lenses and the like is being investigated enthusiastically (non-patent document 1 and patent document 2).
Furthermore, application of zirconia sintered bodies to aesthetic dental parts and the like is being investigated (patent documents 3 and 4), and application thereof to decorative parts and exterior parts is being investigated recently. There hence is a need for a titania-containing zirconia sintered body which has high mechanical strength besides translucency. However, the conventional titania-containing transparent zirconia is insufficient in mechanical strength although translucent.
In patent document 2, for example, a zirconia sintered body which, when examined at a sample thickness of 0.8 mm and a measuring wavelength of 600 nm, has an in-line transmission of 65% has been reported. This zirconia sintered body, however, has a grain size as large as about 100 μm and has low mechanical strength. Non-patent document 2 discloses a titania-containing transparent zirconia sintered body having a grain size of 200 μm, but the mechanical strength (bending strength) thereof is as low as about 210 MPa.
As described above, neither a titania-containing zirconia sintered body which combines mechanical strength and high translucency nor a process for producing the sintered body has been known.
Meanwhile, in the case where a zirconia sintered body is to be used in optical material applications, the sintered body is required to have a low transmission loss and exceedingly high transparency.
The transparent zirconia sintered bodies which have been reported so far include a transparent zirconia sintered body which contains 6 mol % or more yttria and 3 mol % or more titania and has an in-line transmission of 66% (sample thickness, 1 mm; measuring wavelength, 600 nm) and a transparent zirconia sintered body which contains 7.2 mol % yttria and 10 mol % titania and has an in-line transmission of 72% (sample thickness, 1 mm) (patent documents 2, 4, and 5 and non-patent document 1).
Furthermore, zirconia sintered bodies which contain 6-15 mol % yttria and have an in-line transmission of 73% (sample thickness, 1 mm; measuring wavelength, 550 nm) have been reported (patent document 1).
However, those sintered bodies are dimmed, i.e., so-called cloudy sintered bodies, although having some degree of translucency, and higher transparency has been necessary for optical applications.
Moreover, in the production process disclosed in the cited document 1, the production atmosphere is regulated on the basis of a method for disposing the sample vessel. It has hence been necessary that the atmosphere should be stably regulated in high-temperature HIP, which is accompanied with a deformation of the vessel, etc.
As described above, there has been neither a zirconia sintered body which has high translucency and undimmed high transparency nor a process for zirconia sintered body production which can yield such sintered body with satisfactory reproducibility.